The present invention relates to the high power x-ray tube arts. It finds particular application in conjunction with x-ray tubes for CT scanners and will be described with particular reference thereto. It is appreciated, however, that the invention will also find application in conjunction with other types of vacuum tubes employing high power cathodes and temperature sensitive anodes.
In early x-ray tubes, electrons from a cathode filament were drawn at a high voltage to a stationary target anode. The impact of the electrons caused the generation of x-rays as well as significant thermal energy. As higher power x-ray tubes were developed, the thermal energy became so large that extended use tended to damage the anode.
Today, one of the principal ways to distribute the thermal loading and reduce anode damage is to use a rotating anode. The electron beam is focused near a peripheral edge of an anode disk. As the anode rotates, the portion of the anode where x-rays are generated moves along an annular path. Each spot along the annular footprint is heated to a very high temperature as it passes under the electron beam and cools as it rotates around before returning for the generation of additional x-rays. However, if the path of travel is too short, the target area on the anode can still contain sufficient thermal energy that the additional thermal energy from the electron beam can still cause thermal damage to the anode surface. Thus, as higher power x-ray tubes are developed, the diameter and the mass of the anode continues to grow. Unfortunately, this growth has undesirable side effects, such as increasing x-ray tube cost, greater tube size, more massive tube mounting assemblies, and the like. These problems are particularly acute in CT machines where space is very tight.
An additional cost, heretofore unrecognized, is incurred by the inefficient use of the anode surface area. Recall that the path etched on a rotating anode by the electron beam is a linear ring. This results in a very small relative portion of the anode surface ever being struck by electrons for the generation of x-rays, essentially using the large remainder only for absorption of thermal energy.
Other costs are incurred from the use of less efficient heat exchanging methods. In today's rotating anode x-ray tubes, cooling is difficult. Recall that a bearing mounted rotating anode is located in a vacuum and that the impact of electrons causes significant thermal energy in addition to x-rays. In order to protect the anode, various methods to reduce or dissipate the thermal energy have been used. There are three generally accepted ways to transfer heat energy; namely, convection, conduction and radiation.
Concerning present x-rays tubes, two of these methods lack efficacy. Convection is ineffective due to the vacuum in which the anode is typically located. Conduction is limited due to the bearings on which rotating anodes are mounted. In a rotating anode x-ray tube, the conduction path is typically through the bearing on which the anode is mounted. Not only does the passage of heat through a bearing degrade it, but the conduction is slower than the rate at which energy is added. The circulation of cooling fluid through the bearing would cause fluid and vacuum sealing difficulties. Thus, in rotating anode x-ray tubes, radiation heat exchange is the primary way of transferring heat energy to oil circulating around the exterior of the vacuum envelope.
The present invention contemplates a new, improved x-ray tube configuration and method of x-ray generation which overcomes the above difficulties and others.